Effect of low temperature exposure on tensile mechanical properties of carbon fiber/epoxy composites
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摘要: 针对低温暴露对碳纤维/环氧树脂(CF/EP)复合材料力学性能影响进行研究，对低温0℃、−20℃、−40℃、−60℃暴露100 h、200 h、300 h、400 h、500 h后，对CF/EP的复合材料拉伸力学性能影响展开研究，利用SEM电镜扫描分析损伤机制，根据试验结果提出了一种预测CF/EP复合材料低温暴露后剩余强度的预测公式。试验结果表明，在长时间低温暴露后，CF/EP复合材料拉伸强度随低温暴露时间的增长呈现出先增后降的趋势；低温暴露时间低于300 h时，CF/EP复合材料拉伸强度随温度下降先增后降，暴露时间高于300 h后，拉伸强度随温度下降逐渐降低；CF/EP复合材料拉伸弹性模量随低温暴露时间的增长呈现逐渐上升趋势，温度越低，上升趋势越明显。SEM结果表明，低温暴露后，纤维与环氧树脂黏结程度增强，有利于荷载传递，CF/EP复合材料拉伸强度增大，破坏形貌上表现为纤维上包裹更多树脂；长时低温暴露后，由于纤维与基体收缩系数不同导致微裂纹产生，在受到荷载时裂纹进一步扩散，不利于荷载传递，使拉伸强度下降，破坏形貌上表现为纤维成束凝集，纤维束间距增大。基于初始试验，本文提出了一种基于初始试验的CF/EP复合材料低温暴露后剩余强度预测模型，试验与预测结果吻合较好，由于考虑了同种材料在不同低温和暴露时间耦合作用下的等效作用，可减少相同材料在不同低温与暴露时间下的试验次数，因此具备一定参考价值。Abstract: According to the effects of low temperature exposure on the mechanical properties of carbon fiber/epoxy (CF/EP) composites, the tensile properties of TR50S/YPH-69 were selected after low temperature of 0℃, −20℃, −40℃, −60℃ for 100 h, 200 h, 300 h, 400 h and 500 h to investigate the effects of low temperature, then the damage mechanism of material was analyzed by SEM. Based on the test results, a prediction formula was proposed to predict the residual strength of CF/EP composites exposed at low temperature. The test results present that after experiencing long time cryopreservation, the tensile mechanical properties of CF/EP composites increase first and then decrease, with the increase of the low-temperature exposure time. When the exposure time at low temperature is less than 300 h, the tensile strength of CF/EP composites increases first and then decreases as the temperature decreases; after the exposure time exceeds 300 h, the tensile strength gradually decreases as the temperature decreases. And the tensile elastic modulus of CF/EP composites shows a gradual upward trend with the increase of low-temperature exposure time, the lower the temperature, the more obvious the upward trend. SEM analysis shows that after a short period of low temperature exposure, the fiber and epoxy resin bond stronger, which is conducive to load transfer and enhance the CF/EP composites’s tensile ability, the destruction morphology is represented by more resin wrapped on the fiber. After long-term exposure to low temperature, material may occur cracks due to different contraction coefficient. Under the applied load, the crack gets further spread, which is not conducive to the load transfer, and causes the tensile strength to decline. The failure morphology shows that the fibers agglomerate into bundles, and the fiber bundle spacing increases. Based on initial test, a formula of predicting the residual strength of CF/EP composites after low temperature exposure was proposed. The test results are in good agreement with the predicted results. The number of tests can be reduced, due to consider the equivalent effect of the same material under different low temperatures and exposure time.
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Key words:
- carbon fiber /
- low temperature environment /
- tensile property /
- damage mechanism /
- strength prediction
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图 3 CF/EP复合材料不同低温暴露100 h、200 h、300 h、400 h、500 h(从左到右)后拉伸破坏典型形貌
Figure 3. Typical morphology of tensile failure after CF/EP composites exposure at different low temperatures for 100 h, 200 h, 300 h, 400 h, 500 h (from left to right)
图 4 CF/EP复合材料拉伸力学实验结果
Figure 4. Tensile mechanics experiment results of CF/EP composites
图 5 CF/EP复合材料受低温暴露作用强度变化趋势
Figure 5. Variation of CF/EP composites tensile strength after low temperature exposure
图 6 CF/EP复合材料拉伸强度随暴露温度降低和暴露时间增长变化
Figure 6. CF/EP composites tensile strength changes with decreasing exposure temperature and increasing exposure time
图 7 CF/EP复合材料受低温暴露作用弹性模量变化趋势
Figure 7. Variation of CF/EP composites elastic modulus after low temperature exposure
图 8 CF/EP复合材料弹性模量随暴露温度降低和暴露时间增长变化
Figure 8. CF/EP composites modulus changing with decreasing exposure temperature and increasing exposure time
图 9 低温暴露前后CF/EP复合材料试样拉伸破坏的横截面断口SEM图像
Figure 9. SEM images of cross-section fractures of CF/EP composite specimens before and after low temperature exposure
表 1 碳纤维/环氧树脂(CF/EP)复合材料基本力学性能
Table 1. Basic mechanical properties of carbon fiber/epoxy (CF/EP) composites
Density/(g·cm−3) Tensile strength/MPa Tensile modulus/GPa Elongation at break/% TR50S 1.82 4 900 240 2 YPH-69 1.25 80 3 4 
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表 2 CF/EP复合材料低温暴露试验组安排
Table 2. Arrangement of CF/EP composites low temperature exposure test group
Specimen group
labelingTemperature/
℃Exposure
time/hNumber of
specimenSpecimen group
labelingTemperature/
℃Exposure
time/hNumber of
specimens− 25 − 5 − − − − O 0 a-100 5 D −40 a-100 5 b-200 5 b-200 5 c-300 5 c-300 5 d-400 5 d-400 5 e-500 5 e-500 5 B −20 a-100 5 F −60 a-100 5 b-200 5 b-200 5 c-300 5 c-300 5 d-400 5 d-400 5 e-500 5 e-500 5
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表 3 CF/EP复合材料低温环境暴露等效作用时间
Table 3. Equivalent times of CF/EP composites low temperature environment exposure
0℃/273 K −20℃/253 K −40℃/233 K −60℃/213 K 0 h 0 0 0 0 100 h 91.61 84.9 78.2 71.5 200 h 183.2 169.8 156.4 143.0 300 h 274.8 254.7 234.6 214.4 400 h 366.4 339.6 312.8 285.9 500 h 458.1 424.5 391.0 357.4
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表 4 不同低温条件的公式参数值数值计算结果
Table 4. Numerical calculation results of formula parameter values under different low temperature conditions
$ \eta $ $ {\lambda }_{i} $ $ \beta $ $ {\theta }_{i} $ 1 0℃/273 K 7 200 0.00197 4 600 0.00339 2 −20℃/253 K 7 200 0.00262 4 600 0.00485 3 −40℃/233 K 7 200 0.00274 4 600 0.00555 4 −60℃/213 K 7 200 0.00286 4 600 0.00645 Notes: $ \eta $—Significant parameter of low temperature strengthening effect on the material, the same material is a certain value; $ \beta $—Significant parameter of crack propagation resistance of the material, and the same material is a certain value; $ {\lambda }_{i} $ and $ {\theta }_{i} $—State parameter of the material in the low-temperature environment and the influence parameter of the low-temperature environment, which varies according to the low-temperature environment of the material; $ i $—Low temperature, $ i $=1 (0℃), 2 (−20℃), 3 (−40℃), 4 (−60℃).
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表 5 不同试验条件下CF/EP复合材料拉伸强度试验值与预测值对比
Table 5. Comparison of experimental and predicted values of CF/EP composites tensile strength under different test conditions
T/℃ t/h S/MPa Si/MPa Si−S Relative tolerance/% −20 300 2 324.48 2 291.83 −32.65 −1.40 400 2 236.28 2 270.27 33.99 1.52 500 2 196.49 2 187.09 −9.40 −0.43 −40 300 2 301.16 2 272.89 −28.27 −1.23 400 2 236.17 2 236.93 0.76 0.03 500 2 138.15 2 136.39 −1.76 −0.08 −60 300 2 272.21 2 225.25 −46.96 −2.07 400 2 133.13 2 171.08 37.95 1.78 500 2 064.56 2 050.51 −14.05 −0.68 Notes: T—Temperature; t—Exposure time; S—Test values of tensile strength; Si—Predicted values of tensile strength.
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